Not sure if either has been mentioned yet, but The Moon is a Harsh Mistress and Time Enough for Love by Robert Heinlein are both really great. My favorite novels by him. Oh yes, forgot about Starship Troopers. That one really got me thinking.

And of course, Frank Herbert's novels are mostly good too. The Dune series (NOT the newer ones by his son and that Anderson fellow) is a given, and I also enjoyed Hellstrom's Hive. Some of his more unknown works are extremely difficult to find.

I read alot of Harry Turtledove. I like Guns of the South, Worldwar series and the American Empire story arc set from 1861-1944 (South won Civil War).

S.M. Stirling writes great alternative history too. His "Drakon" (I think that is how it is spelled) series is great. In it the Southern armies leave the south when the north wins and sail to South Africa and take that area over. I think there are 3-4 books in the series, can't remember exactly. Pretty racy stuff in them though!! Ack!!

I like this thread and I have to admit that I am posting this before reading the other posts so forgive me if I repeat anything. My dad used to have a wall of older sci-fi and when he got sick and I was away from home my loving siblings got rid of everything! At University I actually took one of the most interesting courses of my academic career: sci-fi. So here is my short list and again I apologize for repeats.Blade Runner & UBIK Philip K DickFahrenheit 451 Ray BradburyJohnny Mnemonic William Gibson1984 George OrwellHitch Hikers Guide to the Galaxy Douglas AdamsThe Sirens of Titan Kurt Vonnegut

This last one was actually assigned in a Ethics of Science classStar Trek: Science Logs Andre Bormanis
Its really cool, it shows you the real science in all the Star Trek's.